Schrodinger's paradox

This prompts Alice to exclaim: "Well! I've often seen a cat without a grin, but a grin without a cat! It's the most curious thing I ever saw in my life!"

In the world familiar to us, an object and its properties are always bound together. A rotating ball, for instance, cannot become separated from its spin.

Image copyrightL.FILTERImage caption
The cat (the neutron) goes via the upper beam path, while its grin (the magnetic moment) goes via the lower

But quantum theory predicts that a particle (such as a photon or neutron) can become physically separated from one of its properties - such as its polarisation or its magnetic moment (the strength of its coupling to an external magnetic field).

The feline analogy is a nod to Schrodinger's Cat - the infamous thought experiment in which a cat in a box is both alive and dead simultaneously - illustrating a quantum phenomenon known as superposition.

To prove that the Cheshire Cat is not just a cute theory, the researchers used an experimental set-up known as an interferometer, at the Institute Laue-Langevin (ILL) in Grenoble, France.

A neutron beam was passed through a silicon crystal, sending it down two different paths - like passengers and their luggage at airport security.

By applying filters and a technique known as "post-selection", they were able to detect the physical separation of the neutrons from their magnetic moment - as measured by the direction of their spin.

"The system behaves as if the neutrons go through one beam path, while their magnetic moment travels along the other," the researchers reported.

Image copyrightILLImage caption
The high flux neutron source at the ILL made the weak signal of the 'Cheshire Cat' detectable

Glimpsing this Cheshire Cat requires what quantum physicists call "weak measurement," whereby you interact with a system so gently that you avoid collapsing it from a quantum state to a classical one.

Their delicate apparatus could have useful applications in high-precision metrology, the researchers say.

"For example, one could imagine a situation in which the magnetic moment of a particle overshadows another of the particle's properties which one wants to measure very precisely.

"The Cheshire Cat effect might lead to a technology which allows one to separate the unwanted magnetic moment to a region where it causes no disturbance to the high-precision measurement of the other property."